a. Field of invention
The invention relates to radial power divider/combiners and, in particular, to radial power divider/combiners that are suitable for use in solid-state power-amplifier (SSPA) devices.
b. Background of the invention
Solid State Power Amplifiers (SSPAs) are used in a variety of applications ranging from satellites, radar, and other RF applications requiring high output power. Typical SSPAs can achieve signal output levels of more than 10 watts using solid-state amplifiers such as Monolithic Microwave Integrated Circuits (MMICs), or individual tube amplifiers.
A fundamental problem with conventional SSPA technology is that individual MMICs produce less power and operate at lower efficiency compared to the individual tube devices. At Ka-band, for example, currently available MMIC chips have output power capability that is approximately an order of magnitude less compared to the Traveling Wave Tube Amplifier (TWTA). The efficiency is approximately half.
Although a single MMIC amplifier chip cannot achieve the requisite level of output power without excessive size and power consumption issues, MMIC technology is far more practical than tubes in space and other applications. MMIC technology offers a reduction in supply voltage, potential reduction in cost, improvement in linearity and reliability.
Consequently, efforts have been made to combine the outputs of several individual MMIC amplifiers to achieve the desired total transmitter output, and it has been found that a combination of a large number of MMICs is attractive for applications where these advantages outweigh the lost efficiency. Consequently, existing SSPA designs using MMIC chips typically use a radial splitting and combining architecture in which a signal is divided into a number of individual components. Each individual signal component is amplified by a respective amplifier, and the outputs of the amplifiers are combined into a single output that achieves the desired overall signal amplification.
However, to meet the output power requirements of space telecommunication systems, it is necessary to power combine a large number of individual MMICs in the SSPA, and yet this must be done in a highly efficient manner.
Existing power-combiners such as the in-phase Wilkinson combiner or the 90-degree branch-line hybrid combine a number of binary combiners in a cascaded manner, but this architecture becomes very lossy and cumbersome when the number of combined amplifiers becomes large. For example, to combine eight amplifiers using a conventional, binary microstrip branch-line hybrid at Ka-band (about 26.5 GHz), the combiner microstrip trace tends to be about six inches long and its loss tends to exceed 3 dB. A 3 dB insertion loss infers that half of the RF power output is lost, and this is unacceptable for most applications.
To overcome these loss and size problems, other approaches including the stripline radial combiner, oversized coaxial waveguide combiner, and quasi-optical combiner, have been investigated. The stripline radial combiner, using multi-section impedance transformers and isolation resistors, still suffers excessive loss at Ka-band, mainly because of the extremely thin substrate (<10 mil) required at Ka-band. The coaxial waveguide approach uses oversized coaxial cable, which introduces moding problems and, consequently, is useful only at low frequencies. The quasi-optical combiner uses hard waveguide feed horns at both the input and output to split and combine the power, and these are very large and cumbersome.
United States Patent Application 20050174194 by Wu, You-Sun et al. published Aug. 11, 2005 shows an N-Way Radial Power Divider/Combiner in which an input signal is provided to a transmission antenna that propagates into a divider. Within the divider, the input signal is divided into a plurality N of individual signals by waveguides disposed in a radial configuration around the transmitting antenna such that at least a portion of the input signal radiated by the antenna enters an input end of each waveguide. The individual signals are received by receiving antennas and provided to respective amplifiers. The amplifiers amplify the respective individual signals by a desired amplification factor. The amplified individual signals are provided to a plurality of transmitting antennas within the combiner. Inside the combiner, the amplified individual signals are combined to form an output signal that is received by a receiving antenna in the combiner. Though a ten-way divider/combiner is shown, N is said to be in the range of two to 100. The overall insertion loss of the 10-way power divider-combiner was measured using input signals from 20 to 30 GHz, and at 26.5 GHz, the loss for the combiner alone is 0.71 dB at 26.5 GHz.
It would be desirable to adapt a radial power-combiner architecture similar to the foregoing for a higher frequency bandwidth to power combine a larger number of amplifiers with better efficiency, using a smaller combining circuit that has minimum power loss. This is herein achieved by increasing the number of combining ports using reduced height waveguides in the radial base. The radial base has reduced-height waveguides with rectangular waveguide inputs leading a circular waveguide output, defining properly spaced and properly chosen waveguide steps having incremental height changes. The reflections from the walls of the reduced height waveguides are matched by a matching post coupled to a “Marie” mode transducer. The present invention provides a low-loss, compact radial power divider/combiner for use in high-frequency SSPAs that offers an unparalleled size, weight, and power combination, thereby offering a replacement for tube-based flight and ground amplifiers used in earth-orbiting defense missions and radar applications, as well as satellite secure communications systems requiring large bandwidths (secure satellite uplinks, downlinks, and cross-links), etc.